A
bstract
A search for the supersymmetric partners of quarks and gluons (squarks and gluinos) in final states containing jets and missing transverse momentum, but no electrons or muons, is presented. ...The data used in this search were recorded by the ATLAS experiment in proton-proton collisions at a centre-of-mass energy of
s
= 13 TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb
−
1
. The results are interpreted in the context of various
R
-parity-conserving models where squarks and gluinos are produced in pairs or in association and a neutralino is the lightest supersymmetric particle. An exclusion limit at the 95% confidence level on the mass of the gluino is set at 2.30 TeV for a simplified model containing only a gluino and the lightest neutralino, assuming the latter is massless. For a simplified model involving the strong production of mass-degenerate first- and second-generation squarks, squark masses below 1.85 TeV are excluded if the lightest neutralino is massless. These limits extend substantially beyond the region of supersymmetric parameter space excluded previously by similar searches with the ATLAS detector.
A search for singly produced vector-like
Q
quarks, where
Q
can be either a
T
quark with charge
+
2
/
3
or a
Y
quark with charge
-
4
/
3
, is performed in proton–proton collisions recorded with the ...ATLAS detector at the LHC. The dataset corresponds to an integrated luminosity of 20.3 fb
-
1
and was produced with a centre-of-mass energy of
s
=
8
TeV. This analysis targets
Q
→
W
b
decays where the
W
boson decays leptonically. A veto on massive large-radius jets is used to reject the dominant
t
t
¯
background. The reconstructed
Q
-candidate mass, ranging from 0.4 to 1.2 TeV, is used in the search to discriminate signal from background processes. No significant deviation from the Standard Model expectation is observed, and limits are set on the
Q
→
W
b
cross-section times branching ratio. The results are also interpreted as limits on the
QWb
coupling and the mixing with the Standard Model sector for a singlet
T
quark or a
Y
quark from a doublet.
T
quarks with masses below 0.95 TeV are excluded at 95 % confidence level, assuming a unit coupling and a BR
(
T
→
W
b
)
=
0.5
, whereas the expected limit is 1.10 TeV.
Charged-particle fragmentation functions for jets azimuthally balanced by a high-transverse-momentum, prompt, isolated photon are measured in 25 pb(-1) of pp and 0.49 nb(-1) of Pb + Pb collision data ...at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb + Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photon-tagged jets are observed to have larger modifications than inclusive jets in central Pb + Pb events.
This Letter presents a search for new light resonances decaying to pairs of quarks and produced in association with a high-pT photon or jet. The dataset consists of proton-proton collisions with an ...integrated luminosity of 36.1 fb-1 at a centre-of-mass energy of s√=13 TeV recorded by the ATLAS detector at the Large Hadron Collider. Resonance candidates are identified as massive large-radius jets with substructure consistent with a particle decaying into a quark pair. The mass spectrum of the candidates is examined for local excesses above background. No evidence of a new resonance is observed in the data, which are used to exclude the production of a lepto-phobic axial-vector Z' boson.
A search for the associated production of a heavy resonance with a top-quark or a top-antitop-quark pair, and decaying into a
t
t
¯
pair is presented. The search uses the data recorded by the ATLAS ...detector in
pp
collisions at
s
=
13
TeV
at the Large Hadron Collider during the years 2015–2018, corresponding to an integrated luminosity of 139
fb
-
1
. Events containing exactly one electron or muon are selected. The two hadronically decaying top quarks from the resonance decay are reconstructed using jets clustered with a large radius parameter of
R
=
1
. The invariant mass spectrum of the two top quark candidates is used to search for a resonance signal in the range of 1.0
TeV
to 3.2
TeV
. The presence of a signal is examined using an approach with minimal model dependence followed by a model-dependent interpretation. No significant excess is observed over the background expectation. Upper limits on the production cross section times branching ratio at 95% confidence level are provided for a heavy
Z
′
boson based on a simplified model, for
Z
′
mass between 1.0
TeV
and 3.0
TeV
. The observed (expected) limits range from 21 (14) fb to 119 (86) fb depending on the choice of model parameters.
A
bstract
An analysis is presented of events containing jets including at least one
b
-tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric ...charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb
−1
of
pp
collisions at
s
=
8
TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events; the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral
b
′-quarks, and production of two positively charged top quarks. Eleven signal regions are defined; subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral
b
′-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.
A
bstract
A search for long-lived particles, which have come to rest within the ATLAS detector, is presented. The subsequent decays of these particles can produce high-momentum jets, resulting in ...large out-of-time energy deposits in the ATLAS calorimeters. These de- cays are detected using data collected during periods in the LHC bunch structure when collisions are absent. The analysed dataset is composed of events from proton-proton collisions produced by the Large Hadron Collider at a centre-of-mass energy of
s
= 13 TeV and recorded by the ATLAS experiment during 2017 and 2018. The dataset used for this search corresponds to a total live time of 579 hours. The results of this search are used to derive lower limits on the mass of gluino
R
-hadrons, assuming a branching fraction
B
g
~
→
q
q
¯
χ
1
0
= 100%, with masses of up to 1
.
4 TeV excluded for gluino lifetimes of 10
−
5
to 10
3
s.
Searches for new resonances in the diphoton final state, with spin 0 as predicted by theories with an extended Higgs sector and with spin 2 using a warped extra-dimension benchmark model, are ...presented using 139 fb−1 of s=13 TeV pp collision data collected by the ATLAS experiment at the LHC. No significant deviation from the Standard Model is observed and upper limits are placed on the production cross-section times branching ratio to two photons as a function of the resonance mass.
An updated search is performed for gluino, top squark, or bottom squark R-hadrons that have come to rest within the ATLAS calorimeter, and decay at some later time to hadronic jets and a neutralino, ...using 5.0 and 22.9 fb super(-1) of pp collisions at 7 and 8 TeV, respectively. Candidate decay events are triggered in selected empty bunch crossings of the LHC in order to remove pp collision backgrounds. Selections based on jet shape and muon system activity are applied to discriminate signal events from cosmic ray and beam-halo muon backgrounds. In the absence of an excess of events, improved limits are set on gluino, stop, and sbottom masses for different decays, lifetimes, and neutralino masses. With a neutralino of mass 100 GeV, the analysis excludes gluinos with mass below 832 GeV (with an expected lower limit of 731 GeV), for a gluino lifetime between 10 mu s and 1000 s in the generic R-hadron model with equal branching ratios for decays to (ProQuest: Formulae and/or non-USASCII text omitted) and (ProQuest: Formulae and/or non-USASCII text omitted). Under the same assumptions for the neutralino mass and squark lifetime, top squarks and bottom squarks in the Regge R-hadron model are excluded with masses below 379 and 344 GeV, respectively.
A
bstract
A simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in
t
-channel single-top-quark production is presented. This analysis is based on ...data from proton–proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb
−
1
, collected with the ATLAS detector at the LHC. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being
b
-tagged. Stringent selection requirements are applied to discriminate
t
-channel single-top-quark events from the background contributions. The top-quark and top-antiquark polarisation vectors are measured from the distributions of the direction cosines of the charged-lepton momentum in the top-quark rest frame. The three components of the polarisation vector for the selected top-quark event sample are
P
x
′
= 0
.
01
±
0
.
18,
P
y
′
=
−
0
.
029
±
0
.
027,
P
z
′
= 0
.
91
±
0
.
10 and for the top-antiquark event sample they are
P
x
′
=
−
0
.
02
±
0
.
20,
P
y
′
=
−
0
.
007
±
0
.
051,
P
z
′
= 0
.
79
±
0
.
16. Normalised differential cross-sections corrected to a fiducial region at the stable-particle level are presented as a function of the charged-lepton angles for top-quark and top-antiquark events inclusively and separately. These measurements are in agreement with Standard Model predictions. The angular differential cross-sections are used to derive bounds on the complex Wilson coefficient of the dimension-six
O
tW
operator in the framework of an effective field theory. The obtained bounds are
C
tW
∈
−
0
.
9
,
1
.
4 and
C
itW
∈
−
0
.
8
,
0
.
2, both at 95% confidence level.
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